An incident-light phase grating may be used in a photoelectric position measuring device for measuring the relative position of two objects that are movable in relation to one another. The incident-light phase grating should have a high diffraction efficiency and thus the highest possible reflectance of the reflector layers therein, so that the measurement signal is as large as possible in relation to the noise signals in the corresponding position measuring devices.
European Published Patent Application No. 0 160 784 by the applicant hereof describes a phase grating composed of two reflective layers spaced a distance apart, situated on both sides of a transparent spacer layer, with at least one reflective layer being designed as an amplitude grating. In one exemplary embodiment, a reflective layer is a continuous layer. In the publication cited above, chromium and gold are proposed as conventional materials for the reflective layers.
The incident-light phase grating described in European Published Patent Application No. 0 742 455 by the applicant hereof also discloses chromium or gold as the material for the reflective layers. In addition, it is pointed out there that the corresponding incident-light phase grating can also be manufactured by electron-beam lithography.
European Published Patent Application No. 0 773 458 by the applicant hereof mentions the use of gold, silver, copper or aluminum as materials for the reflective layers in conjunction with conventional photolithographic methods.
Because of the steady increase in requirements pertaining to the resolution, i.e., measurement accuracy of such scales designed as incident-light phase gratings, it has become necessary to further reduce the periodic graduations or structures of these incident-light phase gratings. In certain industrial fields, it is even possible to replace complex laser beam interferometry with position measuring devices using incident-light phase grating with suitably finely structured graduations.
However, there are physical limits to the photolithography methods with regard to the fineness of the structures to be manufactured. The reason therefor is diffraction effects during exposure of the photoresist.
These unwanted diffraction effects are minimized in the manufacture of such scales designed as incident-light phase gratings by the electron-beam lithography method, but a satisfactorily fine and sharp graduation cannot be produced using conventional materials such as gold for the reflective layers.